Re: [Intel-gfx] [PATCH v3 7/9] drm: update global mutex lock in the ioctl handler
From: Daniel Vetter <hidden>
Date: 2021-08-19 11:21:50
Also in:
dri-devel, intel-gfx, linux-media, lkml
On Thu, Aug 19, 2021 at 12:53 PM Desmond Cheong Zhi Xi [off-list ref] wrote:
On 18/8/21 7:02 pm, Daniel Vetter wrote:quoted
On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote:quoted
In a future patch, a read lock on drm_device.master_rwsem is held in the ioctl handler before the check for ioctl permissions. However, this produces the following lockdep splat: ====================================================== WARNING: possible circular locking dependency detected 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U ------------------------------------------------------ kms_lease/1752 is trying to acquire lock: ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 but task is already holding lock: ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: drm_ioctl_kernel+0xfb/0x1a0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (&dev->master_rwsem){++++}-{3:3}: lock_acquire+0xd3/0x310 down_read+0x3b/0x140 drm_master_internal_acquire+0x1d/0x60 drm_client_modeset_commit+0x10/0x40 __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 drm_fb_helper_set_par+0x34/0x40 intel_fbdev_set_par+0x11/0x40 [i915] fbcon_init+0x270/0x4f0 visual_init+0xc6/0x130 do_bind_con_driver+0x1de/0x2c0 do_take_over_console+0x10e/0x180 do_fbcon_takeover+0x53/0xb0 register_framebuffer+0x22d/0x310 __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 intel_fbdev_initial_config+0xf/0x20 [i915] async_run_entry_fn+0x28/0x130 process_one_work+0x26d/0x5c0 worker_thread+0x37/0x390 kthread+0x13b/0x170 ret_from_fork+0x1f/0x30 -> #1 (&helper->lock){+.+.}-{3:3}: lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 intel_fbdev_restore_mode+0x2b/0x50 [i915] drm_lastclose+0x27/0x50 drm_release_noglobal+0x42/0x60 __fput+0x9e/0x250 task_work_run+0x6b/0xb0 exit_to_user_mode_prepare+0x1c5/0x1d0 syscall_exit_to_user_mode+0x19/0x50 do_syscall_64+0x46/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae -> #0 (drm_global_mutex){+.+.}-{3:3}: validate_chain+0xb39/0x1e70 __lock_acquire+0x5a1/0xb70 lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 drm_open+0x64/0x280 drm_stub_open+0x9f/0x100 chrdev_open+0x9f/0x1d0 do_dentry_open+0x14a/0x3a0 dentry_open+0x53/0x70 drm_mode_create_lease_ioctl+0x3cb/0x970 drm_ioctl_kernel+0xc9/0x1a0 drm_ioctl+0x201/0x3d0 __x64_sys_ioctl+0x6a/0xa0 do_syscall_64+0x37/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae other info that might help us debug this: Chain exists of: drm_global_mutex --> &helper->lock --> &dev->master_rwsem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&dev->master_rwsem); lock(&helper->lock); lock(&dev->master_rwsem); lock(drm_global_mutex); *** DEADLOCK *** The lock hierarchy inversion happens because we grab the drm_global_mutex while already holding on to master_rwsem. To avoid this, we do some prep work to grab the drm_global_mutex before checking for ioctl permissions. At the same time, we update the check for the global mutex to use the drm_dev_needs_global_mutex helper function.This is intentional, essentially we force all non-legacy drivers to have unlocked ioctl (otherwise everyone forgets to set that flag). For non-legacy drivers the global lock only ensures ordering between drm_open and lastclose (I think at least), and between drm_dev_register/unregister and the backwards ->load/unload callbacks (which are called in the wrong place, but we cannot fix that for legacy drivers). ->load/unload should be completely unused (maybe radeon still uses it), and ->lastclose is also on the decline.Ah ok got it, I'll change the check back to drm_core_check_feature(dev, DRIVER_LEGACY) then.quoted
Maybe we should update the comment of drm_global_mutex to explain what it protects and why.The comments in drm_dev_needs_global_mutex make sense I think, I just didn't read the code closely enough.quoted
I'm also confused how this patch connects to the splat, since for i915 weRight, my bad, this is a separate instance of circular locking. I was too hasty when I saw that for legacy drivers we might grab master_rwsem then drm_global_mutex in the ioctl handler.quoted
shouldn't be taking the drm_global_lock here at all. The problem seems to be the drm_open_helper when we create a new lease, which is an entirely different can of worms. I'm honestly not sure how to best do that, but we should be able to create a file and then call drm_open_helper directly, or well a version of that which never takes the drm_global_mutex. Because that is not needed for nested drm_file opening: - legacy drivers never go down this path because leases are only supported with modesetting, and modesetting is only supported for non-legacy drivers - the races against dev->open_count due to last_close or ->load callbacks don't matter, because for the entire ioctl we already have an open drm_file and that wont disappear. So this should work, but I'm not entirely sure how to make it work. -DanielOne idea that comes to mind is to change the outcome of drm_dev_needs_global_mutex while we're in the ioctl, but that requires more locking which sounds like a bad idea. Another idea, which is quite messy, but just for thoughts, uses the idea of pushing the master_rwsem read lock down:
Yeah I think that's cleaner, and I think that also should work a lot better for the other ioctls: - We don't have a need to flush readers anymore since we'll just take the rwsem in write mode - There's much less inversions, and maybe we could even get rid of the spinlock since at that point all readers should at least have the rwsem read-locked.
quoted hunk ↗ jump to hunk
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 7f523e1c5650..5d05e744b728 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c@@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = { DRM_RENDER_ALLOW), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0), - DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER), + DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER),diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c index 983701198ffd..a25bc69522b4 100644 --- a/drivers/gpu/drm/drm_lease.c +++ b/drivers/gpu/drm/drm_lease.c@@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, return -EINVAL; } + /* Clone the lessor file to create a new file for us */ + DRM_DEBUG_LEASE("Allocating lease file\n"); + lessee_file = file_clone_open(lessor_file); + if (IS_ERR(lessee_file)) + return PTR_ERR(lessee_file); + + down_read(&dev->master_rwsem); + + if (!drm_is_current_master(lessor_priv)) { + ret = -EACCES; + goto out_file; + } + lessor = drm_file_get_master(lessor_priv); /* Do not allow sub-leases */ if (lessor->lessor) {@@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, goto out_leases; } - /* Clone the lessor file to create a new file for us */ - DRM_DEBUG_LEASE("Allocating lease file\n"); - lessee_file = file_clone_open(lessor_file); - if (IS_ERR(lessee_file)) { - ret = PTR_ERR(lessee_file); - goto out_lessee; - } - lessee_priv = lessee_file->private_data; /* Change the file to a master one */ drm_master_put(&lessee_priv->master);@@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, fd_install(fd, lessee_file); drm_master_put(&lessor); + up_read(&dev->master_rwsem); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); return 0; -out_lessee: - drm_master_put(&lessee); - out_leases: put_unused_fd(fd); out_lessor: drm_master_put(&lessor); + +out_file: + up_read(&dev->master_rwsem); + fput(lessee_file); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret); return ret; }Something like this would also address the other deadlock we'd hit in drm_mode_create_lease_ioctl(): drm_ioctl_kernel(): down_read(&master_rwsem); <--- down_read() drm_mode_create_lease_ioctl(): drm_lease_create(): file_clone_open(): ... drm_open(): drm_open_helper(): drm_master_open(): down_write(&master_rwsem); <--- down_write() Overall, I think the suggestion to push master_rwsem write locks down into ioctls would solve the nesting problem for those ioctls.
Yup, my gut feeling agress. And the above is a nice solution without having to dig out all the code for creating a file directly (it's doable I think at least, we do it for dma-buf).
Although I'm still a little concerned that, just like here, there might be deeply embedded nested locking, so locking becomes prone to breaking. It does smell a bit to me.
Yeah, that's pretty much the bane of locking cleanup/rework. You have to do it to figure out what goes boom :-/ Even with the most careful audit there's surprises left. -Daniel
quoted
quoted
Signed-off-by: Desmond Cheong Zhi Xi <redacted> --- drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-)diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 880fc565d599..2cb57378a787 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c@@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, if (drm_dev_is_unplugged(dev)) return -ENODEV; + /* Enforce sane locking for modern driver ioctls. */ + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) + mutex_lock(&drm_global_mutex); + retcode = drm_ioctl_permit(flags, file_priv); if (unlikely(retcode)) - return retcode; + goto out; - /* Enforce sane locking for modern driver ioctls. */ - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || - (flags & DRM_UNLOCKED)) - retcode = func(dev, kdata, file_priv); - else { - mutex_lock(&drm_global_mutex); - retcode = func(dev, kdata, file_priv); + retcode = func(dev, kdata, file_priv); + +out: + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) mutex_unlock(&drm_global_mutex); - } return retcode; } EXPORT_SYMBOL(drm_ioctl_kernel); --2.25.1
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Linux-kernel-mentees mailing list Linux-kernel-mentees@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees